Study Finds Birth Order is Not a Predictor of Intelligence Within Families

Has your older brother or sister ever told you that birth order affects intelligence? That the first-born is usually smarter than later-born children? The best evidence says they're wrong. A very thorough study conducted jointly by researchers at Ohio State University, University of Oklahoma, and the University of North Carolina-Chapel Hill released way back in 2006, found no correlation between intelligence and birth order once other factors had been taken into consideration.

These researchers, Aaron Wichman, Joseph Lee Rodgers, and Robert MacCallum used data provided from a nationwide sample of nearly 3000 families. On first review, there did seem to be a correlation between lower birth order and intelligence. Most previous studies saw this same correlation, and the myth was born that later birth order meant a likelihood of lower intelligence relative to older brothers and sisters.

Whenever a study like this is done, one has to be very careful in one's conclusions. Is there something else affecting the results that wasn't considered? In this case, the answer is yes. What are some key differences between first-borns and say fourth-born children? Well, for starters, virtually none of the fourth-born children are only children, or have only one sibling, while a very high percentage of first-borns come from such smaller families.

What happens if we look at birth order only in families with the same number of children? Aaron Wichman, the study's lead author and teaching fellow in psychology at Ohio State University said, "In reality, if you look at these larger families, the fourth-born child is just as intelligent as the first-born. But they all don't do as well as children from a smaller family." He goes on to say "larger families have disadvantages that will impact children's intelligence." This is not to say that every small family has smarter children than every large family, just that there is a higher chance in larger families that the children will be affected by other factors that may have a limiting factor on their development. These factors might include things like the larger family more likely to be poor than a smaller family. Poverty can influence the quality of the child's diet or medical care. There are certainly other factors too.

The families in the study were followed from 1986-1998. During that time the children were given tests that measured reading recognition, reading comprehension, and mathematical skill. Results were correlated by the age of the child to eliminate this as a possible confounding factor, a step not taken by many earlier tests.

Although the researchers did look at family size as the major correlating factor to a child's intelligence, they found that this introduced a number of other variables. In order to ensure the statistical validity of their results, they settled upon a substitute variable as the main correlating factor: the mother's age at the birth of the first-born. They found that the younger the mother at this point, the higher the likelihood of larger families, less maternal education, and lower socio-economic status. As Wichman put it, "Mother's age encapsulates many variables that could negatively affect the child-rearing environment. The younger a mother was at the birth of her first child, the lower we would expect intelligence scores to be within a family."

In short, children within the same family tend to have similar scores on intelligence tests. According to Wichman, "There is no evidence of any significant association between birth order and intelligence. It's not your birth order that is important - family environment and genetic influences are the really important factors." So when your older sibling starts to throw the old birth order myth in your face, tell them they aren't so smart after all.

Source:

Ohio State University (2006, April 12). Older Children Not Smarter Than Their Younger Sibs, Study Finds. http://researchnews.osu.edu/archive/biorder.htm